Patent GB-1426745-A (GB)
Title: Laser tracking system
IPC: G01S17/02
Filed: 1972-08-29
Abstract: 1426745 Optical tracking DEFENCE SECRETARY OF STATE FOR 25 Oct 1973 [29 Aug 1972] 39946/72 Heading H4D  To enable an unmanned rocket 1, Fig. 1, to dock with a large space vehicle 2 the vehicle 2 is provided with a steerable laser tracking system Fig. 2, comprising a laser 9 which is controlled to always illuminate the rocket 1 and a direction finder determining the position of the illuminated rocket with respect to the axis of the direction finder. Off axis errors in the rocket position cause the transmission of correction signals to the rocket to bring it on to the axis and also steering signals to the laser to cause it to follow the rocket. The image of the rocket is reflected by a mirror 4 on to a nutating mirror 5 such that said image is caused to describe a circle on a 5 x 5 matrix of 25 photodetecting cells 8, Fig. 3. Where the laser is pulsed the image of the rocket is a series of dots 10. The array of detectors is connected to produce 5 column outputs 16 and 5 row outputs 15, Fig. 5A. The position of the centroid of the circularly scanned rocket image is given in both horizontal and vertical directions in terms of a coarse position or datum and a fine position. For each direction there are nine datum i.e. the centre line of each detector and the line between adjacent detectors, these datum being indicated from left to right by the numbers 1-9 in simple binary code. If the circle is partly on one detector and partly on the adjacent detector as in cases 1 and 2, Fig. 7, then the datum is the line between the adjacent detectors. If however the circle is wholly within one detector or is spread over three adjacent detectors as in case 3 then the datum is the centre line of the detector or the centre line of the middle detector of the three. The datum for cases 1, 2 and 3 are represented in the binary code by 0010, 0100, 0111 respectively. The fine position of the circle is determined as the difference between the number of dots instant on two adjacent detectors as in case 1 or case 2 or the difference between the number of dots on  the two outside detectors as in case 3. Thus in  case 1 this measurement is the difference between 12 and 4 i.e. 8 which is then represented in simple binary code i.e. 1000. The full position of the circle in case 1 is then given by an 8 digit binary number comprising the datum followed by the fine measurement i.e. 0010, 1000. In the circuit of Fig. 5A each column of detectors 16 is fed via the threshold amplifier to a counter of the dots on that column such that the four bit count of the dots can then be stored in a respective buffer store 25. Similar circuitry produces stored dot counts for the rows in a store 26 and a duplex switch 27 enables the sequential transmission of all the counts to a grouping circuit 28 whereby a datum generator 29 produces the four bit datum code, a left buffer 30 produces a word indicating the number of dots in the lefthand column or row and a righthand buffer 31 produces a word representing the count in the righthand column or row. The left and righthand words are differenced in an arithmetic unit 32, Fig. 5B and combined with the datum word to produce the combined 8 bit position for the circle of dots. The column and row position words are separated out into two parallel channels by duplexer 33 and then converted into an analogue form by digital to analogue converters 38, 39. The analogue position signals so produced are used to (a) cause the laser beam to track the rocket at 44, 45 and (b) via amplifier 46, 47 for the transmission of correction signals to the rocket. To enable the initial acquisition of the rocket by the laser beam a sequence of binary words representing a scanning path is substituted by search word generator 24 for the position signals, the transmission channels to the rocket being blocked.